Hydrocarbon conversion catalysts containing Group VIII metals, particularly cobalt or nickel, or compounds of these metals such as the oxides and or sulfides, Group VI metals, particularly molybdenum or tungsten, or compounds of these metals, and phosphorus on alumina or silica-stabilized alumina are well-known with regard to their hydrotreating activity.
U.S. Pat. Nos. 3,232,887 and 3,287,280 describe the use of phosphoric acid in the preparation of hydrotreating catalysts to stabilize impregnation solutions containing salts of the Group VI and Group VIII metals so as to more uniformly impregnate the catalyst support. U.S. Pat. No. 3,755,148 describes the use of a P/MoO.sub.3 weight ratio of between 0.1 and 0.25 in the impregnation solution. U.S. Pat. No. 3,629,146 describes the use of hydrogen peroxide and phosphoric acid in the metals solution so as to permit the impregnation of greater amounts of metals onto the catalyst support.
U.S. Patent 3,617,528 describes the mixing of an alumina with a nickel salt and phosphoric acid before extruding, drying, and calcining. The catalyst so prepared contains 5 to 40% of the phosphorus component which is claimed to minimize the formation of nickel aluminate during the calcination step. Our invention differs in that the 1-10% phosphate present is an essential ingredient of the extruded alumina catalyst support and is not used to prevent formation of nickel aluminate. Furthermore, the calcined alumina support containing phosphate is prepared before the addition of any catalytically active metals and has a higher surface area and greater macroporosity than similarly prepared supports not containing phosphate.
Aluminum phosphate is precipitated onto an alumina gel in U.S. Pat. No. 2,441,297 so as to improve the heat stability and mechanical strength of the hydrotreating catalysts prepared with this base. However, no effect of the phosphate on fresh activity is found. An alumina hydrogel is contacted with a solution of phosphate ions prior to drying and calcining in Canadian Pat. No. 950,439. It is claimed to be essential that the phosphate ions be incorporated into the hydrogel before drying in order to influence the oxide structure formed during dehydration. Our invention differs in that the phosphate ions are added to a dried alumina gel resulting in a catalyst support which when processed into a hydrocarbon conversion catalyst exhibits improved activity as compared to conventional catalysts.